In fluid bed particulate processing of hydrocarbon feedstocks, it is the practice to continuously add fresh particulate regularly, usually daily, and to withdraw equilibrium particulates prior to addition of fresh particulates. This provides room for the incoming fresh material.
Because of this procedure, which results in immediate complete mixing, particulates both fresh in performance and low in contaminants (usually nickel, vanadium, iron, copper, and sodium) are unavoidably withdrawn together with particulates which have been in the unit for varying times as long as two or three months or longer and have aged and drastically dropped in performance, while simultaneously accumulating deleterious metal contaminant. The industry has long felt a need to have a means by which old catalyst can be selectively removed without entrainment of fresh catalyst.